Halobacteria are found in nature in evaporating salt water ponds under conditions of intense light and low oxygen saturation. They contain distinctive brightly colored pigments such as the orange-red pigment, bacterioruberin, or patches of "purple membrane". Halobacteria belong to a phylogenetically distinct group of prokaryotic organisms--the "archaebacteria" (Archaea)--that are as distantly related to the eubacteria as they are to the eukaryotes.
Archaebacteria possess some attributes in common with the eukaryotes and the eubacteria, as well as characteristics that are uniquely archaeal. For example, the archaebacteria possess a eukaryotic-like transcription apparatus with a 7-12 subunit RNA polymerase which is immunologically related to eukaryotic RNA polymerase (1) and promoter structures are similar to those of RNA Pol II (2). In contrast, the archaebacteria have prokaryotic cellular morphology and 23S, 16S and 5S rRNAs with the genes encoding the rRNAs arranged into eubacterial-like operons (3). Notably, the archaebacteria are unique in their membrane composition.
Bacteriorhodopsin (BR) is found as the sole protein in specialized crystalline patches of the "purple membrane" in halobacteria. Synthesis of BR is induced by high light intensity and low oxygen tension and the patches of purple membrane can constitute up to 50% of the archaebacterium Halobacterium halobium cell surface area.
BR consists of a complex of one protein (bacterio-opsin) along with the chromophore retinal in a 1:1 stoichiometric ratio (4). This complex is embedded in the lipid matrix as seven transmembrane hydrophobic .alpha.-helices in a trimeric configuration (5). Retinal is covalently attached at lysine at position 216 approximately one-third of the way across the transmembraneous region of one of the .alpha.-helices (6). The complex of bacterio-opsin with retinal was named bacteriorhodopsin (BR). The so-called bop gene encodes the light-driven protein pump bacteriorhodopsin (BR) in H. halobium.
There has been some reported research on expression of endogenous polypeptides in halobacteria (7, 8 and 9).